Investment Securities (Debt securities issued by corporation) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 2,275.4	$ 3,208.3	$ 2,633.8	$ 2,547.3
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	592.5	880.9
Fair value investments	592.5	880.9
AAA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.8	1.6
AA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	60.1	42.6
A | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	275.6	192.5
BBB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	163.9	465.2
BB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	71.9	161.7
B | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 12.2	$ 17.3